Localised mastocytosis at the bone and porous coated interface of a knee prosthesis.

A porous coated tibial plateau component, which was not loose, was removed 6 weeks after operation. An absence of bony ingrowth was demonstrated by histological and microradiographic examination. The radiolucent space was filled by young connective tissue and inflammation was indicated by the presence of a few macrophages or foreign body giant cells; there was also an increased number of mast cells near the metallic beads. Such inflammatory cells have not previously been reported around a firmly fixed joint prosthesis, but they have been associated with metallic particles. Since accumulations of mast cells can interfere with the process of bony ingrowth, further studies of removed porous coated prostheses should be made with adequate histological techniques so that the clinical significance of these cells can be better understood.